1. Technical Field
The present invention generally relates to a heat exchanging element and, particularly, to a heat exchanging element adapted to a fuel cell system and fuel cell systems using the same.
2. Description of the Related Art
A fuel cell generally has the advantages of high efficiency, low noise, non-pollution and so on, so it is a kind of energy technology which is capable of meeting the era trend. Fuel cells usually are classified as several types, such as proton exchange membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC). Taking a direct methanol fuel cell as an example, a membrane electrode assembly (MEA) thereof primarily is consisted of a cathode, an anode and a proton exchange membrane sandwiched between the cathode and the anode. A fuel (i.e., methanol) fed to the anode reacts with a catalyst to generate hydrogen ions and electrons. A half equation of anode reaction is that: CH3OH+H2O→CO2+6H++6e−.
In addition, the electrons created from the anode reaction flow to the cathode via an external circuit, while the hydrogen ions penetrate through the proton exchange membrane to the cathode and react with the electrons and oxygen gas to generate water. A half equation of cathode reaction is that: 6H++6e−+3/2O2→3H2O.
In the fuel cell technology, the higher a reaction temperature of the fuel cell stack which is consisted of a plurality of membrane electrode assemblies, the higher a reaction efficiency of the fuel cell. Accordingly, a design of heat recovery has been proposed in the prior art. In addition, in the prior art, the water generated at the cathode also is recovered to meet the need of the anode reaction.
FIG. 1 is a schematic view of a conventional direct methanol fuel cell system. As illustrated in FIG. 1, the conventional direct methanol fuel cell system 100 includes a fuel cell stack 110, a heat exchanging element 120, a flow guiding unit 130 and a blower 140. The flow guiding unit 130 connects the fuel cell stack 110, the heat exchanging element 120 and the blower 140. The blower 140 is configured to supply an airflow 50. The flow guiding unit 130 is configured to guide the airflow 50 to flow through the heat exchanging element 120 and the fuel cell stack 110 in sequence, so as to supply oxygen gas to the cathode reaction.
In one aspect, the fuel cell stack 110 generates heat energy when it occurs a reaction, and the airflow 50 flowing to the fuel cell stack 110 absorbs the heat energy (Hereinafter, the airflow 50 has flowed through the fuel cell stack 110 will be denoted by 50′). Subsequently, the airflow 50′ is guided to the heat exchanging element 120 by the flow guiding unit 130 and transfers the absorbed heat energy to the heat exchanging element 120 so as to increase a temperature of the heat exchanging element 120. In addition, the airflow 50 flowing from the blower 140 to the heat exchanging element 120 absorbs the heat energy of the heat exchanging element 120, such that a temperature of the airflow 50 which will flow to the fuel cell stack 110 is increased and thus a reaction efficiency of the fuel cell stack 110 is improved.
In another aspect, the airflow 50′ flowing through the fuel cell stack 110 carries the water vapor generated by the cathode reaction. The airflow 50 flowing from the blower 140 to the heat exchanging element 120 acts as a cooling airflow, to assist the water vapor in the airflow 50′ to condense into liquid water at the heat exchanging element 120. As a result, the purpose of water recovery is achieved.
However, because a flow rate of the airflow 50 supplied to and required by the fuel cell stack 110 generally is not high, such that it is difficult to supply enough cooling airflow. As a result, the recovered amount of the liquid water is not capable of meeting the need of the anode reaction. Furthermore, the water vapor in the airflow 50′ need to be firstly condensed into liquid water before the heat energy in the airflow 50′ is transferred to the heat exchanging element 120. Because the temperature of the airflow 50′ is decreased after the condensation of the water vapor, the heat energy transferred to the heat exchanging element 120 is correspondingly decreased. Accordingly, the heat recovery efficiency of the conventional fuel cell system 100 is low.